In recent years, there are involved two serious problems regarding a glass substrate toward realization of a high capacity of a hard disk. One of them is concerned with mechanical characteristics at the time of high-speed rotation such as vibration characteristic and strength; and the other is concerned with removal of a contamination remaining on the glass substrate.
In order to improve the vibration characteristic or strength at the time of high-speed rotation, it is necessary to use a glass substrate having an appropriate glass composition taking into consideration various characteristics such as Young's modulus, specific modulus, specific gravity, thermal expansion coefficient, scratch resistance and fracture toughness. In order to achieve those characteristics, it is known that an alkali aluminosilicate glass of a SiO2—Al2O3—R2O system (R2O represents an alkali metal oxide) is suitable, and in particular, Al2O3 is an effective component for improving mechanical characteristics.
On the other hand, as for the residue on the glass substrate, it is known that a cerium oxide abrasive which is suitably used for glass polishing from reasons, for example, a high polishing rate remains as a contamination. In a manufacturing step of a glass substrate, there may be the case where after the main surface and edge of a circular glass plate cut out from a glass plate is polished with a slurry containing a cerium oxide abrasive, in order to further flatten the main surface, final polishing with a slurry containing a colloidal silica abrasive is performed. At that time, even when the cerium oxide abrasive remains on the main surface, it is removed by the final polishing; however, it may be considered that the cerium oxide abrasive deposited onto the edge remains without being removed and is reattached onto the main surface in a cleaning step after the final polishing.
Under such a background, it is desirable to completely remove the cerium oxide abrasive, and a cleaning liquid containing an inorganic acid and ascorbic acid is proposed (see, for example, Patent Documents 1 and 2). According to this cleaning liquid, the cerium oxide abrasive is dissolved and removed by the action of the inorganic acid and ascorbic acid.
It is also proposed to use a cleaning liquid composed mainly of heated sulfuric acid for cleaning in a final step (see, for example, Patent Document 3).    Patent Document 1: JP-A-2006-99847 (claims)    Patent Document 2: JP-A-2004-59419 (claims)    Patent Document 3: JP-A-2008-90898 (claims)